Illustration of a pollen grain of an elm tree (Ulmus sp.). Each grain is covered with a thin, paper-like hull for easy distribution by the wind. The shape of the grains may vary, but in most cases they are suboblate to spheroidal. The elm tree flowers early in the year before its leaves appear. The pollen is a strong allergen causing allergic reactions including rhinitis and hay fever, and in some people asthma-like symptoms.